This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Multifunctional structural proteins, serving as linkers or adaptors, are essential for cellular remodeling processes. Our studies focus on how molecular interactions of the cytoskeletal protein 4.1R contribute to cell division and to differentiation. Protein 4.1R was initially identified in mature red cells as a multifunctional structural protein in dynamic junctional complexes crucial for red cell skeletal organization and mechanical stability. However, subsequently protein 4.1R was found to be ubiquitously expressed in non-erythroid and erythroid nucleated cells and detected at centrosomes, mitotic spindles, and in nuclei. We reported that 4.1 is required for proper nuclear assembly as well as proper mitotic spindle and centrosome formation in vitro. In intact mammalian cells, we showed that 4.1R depletion by RNA interference compromises centrosomal structure, the fidelity of mitosis and cytokinesis, and alters cell cycle progression. On the basis of our observations, we hypothesize that 4.1 interactions link structural components of centrosomes and nuclei to provide dynamic properties needed for rapid assembly/disassembly during cell division and during differentiation. We previously identified that the spectrin/actin binding domain and the C-terminal domain of 4.1R directly function in assembly of these subcellular structures. Our current goal is to obtain a detailed understanding of 4.1R interactions and mechanisms that contribute to cellular remodeling during assembly/disassembly in cell division and differentiation by identifying 4.1R binding partners and analyzing how 4.1 modulates their functions. We anticipate that identifying functional interactions of protein 4.1 will further increase our understanding of fundamental principles of assembly and dynamics of nuclei, centrosomes and mitotic spindles during cell division and differentiation. This in turn may indicate the genesis of previously unrecognized pathological consequences of 4.1R gene defects. Furthermore, our investigations will contribute a broad perspective on molecular disease loci involving defective or deficient cytoskeletal proteins.